Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.093A.

‘Aishah reports that the Messenger of Allah said: “When one of you becomes drowsy in salah, he should lie down until he is fresh again; otherwise, he will not know if he is asking forgiveness or vilifying himself.” This is related by the group.

Abu Hurairah reports that the Messenger of Allah said: “When one of you gets up at night for salah and his tongue falters in reciting the Qur’an and he is not certain about what he is reciting, he should sleep.” This is related by Ahmad and Muslim.